Interpretive Summary: Growth-promoting implants have been recommended for ruminants for their ability to increase lean tissue accretion, and improve feed conversion efficiency. This project was performed to compare the effects of three U.S. approved anabolic implants (Synovex-S; Ralgro and Revalor-S) on muscle fiber morphology in steers. Since muscle fibers represent the cellular component of lean tissue our interest was if and/or how anabolic implants enable lean tissue accretion. Results indicate that there were significant differences in the performance of the three implants investigated. Synovex-S did not lead to any significant changes in muscle fiber characteristics. Revalor-S was the most effective followed by Ralgro, both exhibiting significant improvements in muscle fiber enlargements which would translate into an increase in lean tissue mass. This information should be very useful for cattle producers who market meat animals.

Technical Abstract:
Growth-promoting implants lead to increased muscle accretion in ruminants. To elucidate the effects at a cellular level, muscle fiber distribution and cross-sectional area (CSA) of longissimus (LM) and semitendinosus (ST) muscles were compared in implanted and control steers. Sixty-four Charolais steers were assigned to one of four treatments (16 steers/treatment): (1) no implant, (2) Synovex-S (estradiol benzoate+progesterone), (3) Ralgro (zeranol) or (4) Revalor-S (trenbolone acetate+estradiol-17B). The experiment was carried out using four slaughter groups (SGRP). Sixteen steers each were slaughtered after 48, 104, 160 and 175 d (four steers/treatment) on trial. Steers on an implant treatment were first implanted at 15 mo of age (day 0) and re-implanted at 56 and 112 d. Muscle fibers were characterized as either slow-twitch oxidative (SO), fast-twitch oxidative-glycolytic (FOG) and fast-twitch glycolytic (FG) fibers. Fiber distribution was minimally affected by SGRP in these physiologically matur steers. Implantation with Synovex did not alter fiber distribution in either muscle compared with control steers. Ralgro- and Revalor-implanted steers had an almost constant fiber distribution in the LM throughout the experiment resulting in higher percentages of FG fibers in SGRP 2 (P<0.05). Fiber area increased in both muscles from SGRP 1 to 3 in all treatment groups followed by a plateau. Synovex did not affect fiber area compared with control steers whereas Ralgro and Revalor implants led to larger fibers in SGRP 3 and 4, respectively. It can be concluded that some growth-promoting implants result in noticeable differences in muscle hypertrophic response which coincide with their different effectiveness to enhance lean mass accretion.